Does the universe lose energy when spacetime expands? When reading about the Big Bang you’ll commonly hear that the universe began “cooling down” soon after. However, if we assume the universe is a closed system, would that thermal energy not continue to exist? If quarks couldn’t bond during the initial phases of the Big Bang, that energy would have to go somewhere. Was it being lost due to the expansion of the universe into empty spacetime?
 A: No. The local energy density is reduced as the universe expands, but since the universe does expand the total energy can simplistically be thought of as remaining the same. Energy is not lost, but is distributed more sparsely. If the universe will enter into a contraction phase after expansion, then the energy density will rise. Nothing will be lost.
I say simplistically because there are two other considerations which should be taken into account.
First, energy is the time component of a vector, and in curved spacetime it only makes sense as a local concept. The total energy of the universe is not a concept with a valid definition.
Second dark energy, if such a thing exists (and one should remember that it is just a speculation, and that there are other explanations for observed phenomena) is locally constant, which means it spontaneously increases as the universe expands.
A: Are you aware of redshift? If a photon with a certain wavelength travels through an expanding universe, the wavelength basically expands while traveling. Since the energy of the photon is inversely proportional to the wavelength it decreases.
Since our universe is dominated by dark energy, which is a constant energy density this energy component is effectively growing (since the universe expands). The energy that the particles lost is then basically turned into gravitational energy.
